The degree of resolution or detail attainable in a reproduced television image ordinarily is limited, not by the transmitted signal, but by the bandwidth of the receiver's luminance channel. As is well known, in the NTSC system luminance information occupies the low frequency portion of the video channel. The high frequency portion is shared by frequency-interleaved luminance and chrominance signal components. Luminance components in this frequency portion represent fine detail in pictures. In conventional television receivers, the luminance bandwidth is typically limited to a range of approximately 2.5 to 3 MHz. A significant improvement in resolution can be obtained by extending the luminance bandwidth to, say, 4 MHz. However, extending the luminance channel bandwidth can result in interference from and beats with chrominance signal components.
D. H. Pritchard et al, describe a system that uses comb filters to remove chrominance signal components from the luminance channel (D. H. Pritchard, A. C. Schoeder, and W. G. Gibson, "A High Performance Television Experiment", "IEEE Transactions on Broadcast and Television Receivers", Vol. 18, No. 2, pp. 82-90 May (1972). Comb filters are especially attractive in such an application because they can be synthesized to have frequency response characteristics corresponding to the interleaved spectra of the NTSC luminance and chrominance signals.
However, comb filtering is for the most part a time domain operation relying on the substantially constant amplitude and phase of the filtered signal on horizontal lines adjacent in time. Consequently, perturbations in the chrominance subcarrier phase or amplitude, such as those that occur during vertical color transistions, will prevent complete cancellation of the chrominance subcarrier. As a result, some unfiltered chrominance signal will be present in the luminance channel. The unfiltered chrominance will cause an objectionable dot pattern to appear in the reproduced television image.